The present invention relates to a spatial light modulator, and more specifically, to a spatial light modulator comprising a solid state light valve formed of a ferroelectric material such as lanthanum modified lead zirconium titanate (PLZT) composition.
A light modulator is a device that modifies some property of a beam of light. A light modulator which operates on the various spatial cross-sectional regions of a light beam is a two dimensional light modulator and is referred to as a "spatial light modulator".
PLZT is a very well known and characterized material. See, for example, an article by Gene H. Haertling, Electronic Ceramics, Chapter 7, (1988). Applications in the present invention employ PLZT of a chemical composition involving a ratio of lanthanum, lead and zirconium which yield a ferroelectric behavior (the rhombohedral or tetragonal structure) and is designated as "memory mode" material. Also, compositions which yield a paraelectric or an antiferroelectric behavior (the cubic or symmetrical structure) are employed and such compositions are designated as "non-memory mode" material. An example of memory mode material could be that designated as 7.5/65/35 (lanthanum/lead/zirconium) and a non-memory mode material could be designated as 9.5/65/35.
Spatial light modulators are well known and extensively studied devices. They are broadly categorized according to the method of impressing upon them the information to be displayed, modulated, etc. The two methods of providing such information to the device are known as "optical" or "electronic" addressing, respectively. Optical addressing is discussed in detail by Jenkins and Tanguay, Neural Networks for Signal Processing, Chapter 9, (1991). Electronic addressing is discussed in detail by Lee and Sheu, Ibid, Chapter 8.
Most spatial light modulators, particularly the optically addressed variety, have a "sandwich" structure in which a photoconductor is sandwiched together with a light modulator material, such as a liquid crystal. The information to be modulated or projected is presented to the spatial light modulator (hereinafter referred to as "SLM") as an optical image and the light valve responds to this image via a photoconductive action which changes the local electric field at the light valve to provide a variance in transmission or reflection. Frequently there is no requirement for a matrix type of electrode structure at all. Electrically addressed spatial light modulators usually require a transverse electrode structure for their operation. The information for such an addressing scheme is provided as a set of electrical signals and are subsequently converted into an image by the modulator structure. This transverse structure is sometimes used to activate a longitudinal or through-the-thickness applied electric field effect in the modulator material.
Some specialized modulators, such as a deformable mirror device, employ a two-dimensional very large scale integration (VLSI) fabricated activation structure formed in/on silicon with a deformable membrane overlying the activation regions.
The initial motivation for the development of light control devices was the projection and flat screen devices for television and computer displays. The next major impetus for the SLM development followed from the discovery of holography and the recognition of the Fourier transforming properties of optical propagation and simple lenses. Accordingly, the number of applications for SLM devices has grown rapidly into such areas as optical signal processing, data processing and computing.
However, there is room for improvement of SLM devices, particularly in the areas of speed of operation, pixel-to-pixel contrast ratio, and illumination uniformity over the display format.